Despite the large numbers of young children whose home language is not English, there is limited systematic research on the course of their English language learning. Poverty only increases the challenges of dual language development in this growing segment of the U.S child population. Lack of information about the normative course of dual language learning raises challenges for early assessment and intervention and has important implications for the health and well-being of these children. Given that Latino children make-up the largest group of English language learners (ELL) in the United States and lag behind both African- Americans and Whites in their educational attainment, understanding their learning of English and Spanish during the preschool period has important public health consequences. The proposed study assesses dual language development in low-income Mexican-origin children who come from predominantly Spanish speaking households. Our objectives are to chart the trajectory of Spanish and English among young low-income children from Spanish-speaking homes when they are exposed to English in the pre-school classroom and to understand classroom variables (e.g., classroom language support and language exposure, and teachers'beliefs) that influence such learning. A total of 45 3-year-old children, their families, and their teachers will be recruited and observed for a year. Their language will be recorded 4 times approximately 3 to 4-months apart. We will observe each child's pre-school classroom to assess teacher talk, language, and literacy support and interview the teacher regarding his/her language skills and beliefs about language development. Our findings will lead to a better understanding of the course of dual language development and the factors that influence it. They will lead to better screening and identification of speech and other learning delays among English language learners, thereby enhancing the health of these at-risk children. The results will also help us understand why some children are more successful in learning English as a second language and will be used for teacher training, parent education, and curriculum development. The study will also provide evidence for the influence of the first and second language on each other and address concerns about the loss of the home language among learners of English as a second language. Finally the study will promote the recruitment and training of minority students for research careers.